22 Apr
2016
22 Apr
'16
11:35 a.m.
{1, 7, 13, 19} is not a group; 7 * 13 = 11.I haven't checked the rest of your list.
My wetware says (7*13)%30 = (91 % 30) = 1.
This is not a big deal if you're generating the groups by hand, but if you're doing it programatically, it means there's something wrong with your program.
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